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Thunda Down Under 2015 - Continued...

This is a continuation from the Thunda Down Under launch. For
Part 1 please see Day 157.

Dark Shadow

We wanted to bring something special to the Thunda Down Under event, and
so we started developing this rocket around 8 months beforehand.
Dark Shadow is our first serious attempt at building a high
altitude water rocket. This rocket was an extension on the lessons
learned with The Shadow and
Shadow II fiberglass rockets. Just
as many have done before to us the main performance upgrade
for this rocket was the use of carbon fiber sleeve reinforcing on the
pressure chamber. A detailed build log kept over
the build period is available here:Dark Shadow
Build Log.

As a part of the development we redesigned Shadow's
deployment mechanism so it was more compact and lighter. The
weight of the entire nosecone including all recovery components
and electronics came down from 380 grams on the Shadow to 270
grams on Dark Shadow. The overall rocket dimensions are similar and the
weight is also comparable. We improved the aerodynamics by
including thinner fins, a boat tail and omitted the rail
buttons. We dropped the nozzle size down to keep the acceleration
to a reasonable level to reduce stress on the deployment
mechanism and the rest of the rocket.

We equipped the payload section with an HD keychain camera, a zLog Mod 6 logging altimeter and an AltimeterOne so we
could correlate the altitude. Parachute deployment
was timer based using our Servo Timer II. We used Dean Wheeler's
simulator to get an estimate of the correct timing. This turned
out to give fairly accurate results.

Because this rocket was untested and we didn't know what may
happen, early on we decided we would build two at the same time
just in case something failed with the first. As it turns out
the second one has about 4% more capacity but is a little
heavier so both have similar simulated performance.

Final Dark Shadow parameters as measured are shown in the
table below. Shadow and Shadow II are included for comparison:

Parameter

Shadow

Shadow II

Dark Shadow #1

Dark Shadow #2

Weight

1370 grams

1440 grams

1550 grams

1620 grams

Diameter

62 mm

62 mm

63 mm

63 mm

Length

2580 mm

2860 mm

2850 mm

2960 mm

Nozzle

19 mm

19 mm

15.5 mm

15.5 mm

Water

1.3 L

1.7 L

1.8 L

1.8 L

Capacity

5.8 L

6.36 L

6.77 L

7.06 L

Flight 1 - Friday 13/3/2015

Flight one was designed as the shakedown flight as there were
a number firsts for us that we hadn't tried before. This
included a CF compression heated pressure chamber under acceleration, new deployment mechanism
under acceleration forces and a new
launcher. For this reason we only pressurised the rocket to
500psi (34.5 bar). The deploy timer was set at 9 seconds.

Because it was quite a hot day ~35C, while getting the rocket
ready out on the pad we placed a number of wet towels over it to help keep it cool. I think this worked fairly
well.

We also used a couple of temporary rail buttons attached
with Velcro that helped guide the rocket straight onto the
launch tube. We wanted as few lateral forces on the nozzle as possible to avoid damage to the pressure chamber/nozzle
interface.

Otherwise set-up and placement on the pad was straight
forward. When it came to arming everything it turned out the
ladder was still too short to easily get at the camera in the
nosecone, and so we pulled down the rocket with the guide rail
and launcher and turned on the electronics that way. After
everything was running we put the rocket and launcher back and locked it into
position. This was a nice feature to have on the quick launcher. A
break-wire trigger also meant that handling the rocket this way
didn't accidentally trigger the timer.

Photos marked with [1] are by Pavel Dvoracek.

Getting launcher ready

Flight line workstation [1]

Prepping with Pavel looking on.

V2 being assembled in
the
background [1]

Getting nosecone ready [1]

Filling with water [1]

Putting onto launch tube [1]

[1]

Arming electronics prior to
launch [1]

Photo time [1]

Alright ... here we go ... [1]

As we pressurised we paused at 200psi for about 30 seconds
and again for a while at 400psi just to give the rocket a chance
to cool down a little. The LCO gave us a 3 count and the launcher released the
rocket easily.

The launch was very fast and straight. We were happy to see
the parachute pop out just past apogee and the rocket drifted to
a safe landing. Everything survived the pressure, acceleration
and landing.

With this flight we also set our new personal best
altitude of 1372 feet (418 m). Unfortunately at the time of
writing we were not able to retrieve the zLog altimeter data so we
only have the AltimeterOne data to go by. Simulated altitude was
1457'. The altimeter has a USB connection problem, but the data
should still be on the unit. We have sent it back to the
manufacturer to see if they can pull the data off via a serial
port. If we get the data back, I will post it here. Never mind
there were more flights to come.

Photo: Phillip Turnbull

Photo: Phillip Turnbull

There she goes ... [1]

About to land

It's a long way down

V2 crew lower left

[1]

Just past apogee

1372 feet on the
AltimeterOne

Local school visit post
landing

Boys doing a great job
explaining how
the rocket works.

Flight 2 - Friday 13/3/2015

We replaced the zLog altimeter with our second one and
prepped the rocket again. First though we had to wash out the dirt that was
pushed into the nozzle on landing . We again used
1.8L of water in the rocket and set the deployment timer delay to 10
seconds.

This time we pressurised the rocket to 600psi (41.4
bar), This was the
target pressure for this project.

The rocket took off well and again the parachute opened
just past apogee.

This time the rocket reached an altitude of 1582 feet (482m) which was an average
of the readings from the AltimeterOne which gave 1587' and the zLog
gave
1575' + 3' (for zero offset) = 1578'. The simulated altitude for this hop was 1732'. The rocket again landed safely under parachute.

Plot from altimeter:

AltimeterOne readout 1587'

The boys were a great help at
the launch

How flat is that?

Making our own vapour clouds

What rocket?

Video screen grab

Just before burnout

Cloud Lasso

Max Q

Safely on the ground

New PB altitude.

Flight 3 - Sunday 15/3/2015

We waited for calmer conditions on Sunday morning to launch
the rocket again. This time the rocket was filled to 640psi
(44.1 bar) and deployment delay was set to 11 seconds. We were
aiming for 650psi, but had launched it just shy of that.

The
rocket went up nicely and reached our new personal best altitude
of 1752 feet (534m). This again is
an average of the two altimeters. AltimeterOne gave a reading of 1761' and
the zLog gave 1747' - 3' (for zero offset) = 1744'. Maximum
speed was 380 feet/s (417km/h) @202' and 0.8 seconds into flight.

We were very happy with that result, as that far exceeded our
expectations. The new altitude is just over 500' (150 m) higher then our
previous best of 1248' ( 380 m) with Shadow II.

Dean Wheeler's simulator predicted an altitude of 1883' @650psi.
We'll go back and adjust the simulator parameters (namely the
drag coefficient) so that it
gives us results closer to the actual and then we'll have a
better model of this rocket for further simulations.

Plot from altimeter:

Ready to crank it up a notch

Calmer conditions in the morning

Cooling rocket prior to launch

And shortly after

On the way up

Flight line and camp site

Apogee

Who put that there?

Retrieving rocket from sorghum
field

1761' on the AltimeterOne

Pulling the payload bay out

Flight performance info
for all 3 flights:

Parameter

Flight #1

Flight #2

Flight #3

Altitude

1372' (418m)

1582' (482m)

1752' (534m)

Top Speed

? km/h

360 feet/s
245mph (395 km/h)

380 feet/s
259mph (417km/h)

Flight Duration

56.5 seconds

65.1 seconds

100.7 seconds

Descent
rate

? m/s

26 feet/s
(7.9 m/s)

19 feet/s
(5.8m/s)

Max acceleration

38G (from sims)

45G (from sims)

49G (from sims)

Combined Altimeter Plot:

Combined Speed Plot:

To top off the great flights, as we
were heading back with the recovered
rocket, we passed
Homer Hickam at the flight line and he said he saw the
flight and liked it, so I asked him if
he could
sign the rocket which he was
happy to do. :) Thank you Homer! So we ended up
with a nice souvenir from the event.

Homer Hickam and Air Command
crew
(Dad is behind the camera)

There is no visible sign of damage or
stress on the rocket which we are happy
about. I think we will
need to see how far this rocket can be
pushed, it probably still has a few psi
up its sleeve, although probably not
much more.

Highlights Video

Here is a highlights video of the 3 Dark Shadow
flights:

Lessons Learned

Fin alignment jig worked really well. There was very
little roll on the way up.

The launcher works well to hold and release the rocket
at these pressures.

The removable rail buttons worked well to hold the
rocket in place while we prepped it and turned on all the
electronics.

Expect dirt to enter the rocket after landing. The
launch tube showed signs of grains of sand scratching the
tube after the first launch.

The deployment mechanism worked 3 out of 3 flights and
deployed the parachute well. The gripper arms were not damaged.

The pressure chamber construction technique worked well.

The nozzle and airframe survived the landings.

Other Flights

We also launched one of our regular water rockets just for
fun. Since we were in Queensland we had to fly at least one
rocket powered by XXXX Gold, The rocket performed as well as
expected but there were a lot of sad thirsty faces at the flight
line on a hot day. If it is any consolation the XXXX was warm
when it went in the rocket. :)

We also flew John's new rocket Dust Devil on a B6-4 and then again on a C6-5.
We also flew Paul's Pod 2 in a two stage configuration boosting
on a C6-0 and staging to C6-5 for a great flight. Both stages
were recovered in good condition ready to fly again.